[0001] The present invention relates to a color imaging apparatus using a solid-state image
pickup device as, for example, an image pickup device for converting an optical signal
to an electrical signal.
[0002] In conventional color imaging apparatuses, most of them use an image pickup tube
(camera tube) as an image pickup device. However, recently, a color imaging apparatus
using a solid-state image pickup device such as a CCD (charge coupled device) type
image pickup device has been developed.
[0003] In the color imaging apparatus using a solid-state image pickup device, a photo sensitive
section for storing charges responsive to the strength of light can be formed in each
pixel. Due to this, as a color filter array which is arranged on the side of the incident
light section of the image pickup device, it is possibleto use arrays of various formats
in which the kind and arrangement of color filters differ. Thus, thus makes it possible
to use a newcolorfilterarraywhich could not be used in the color imaging apparatus
employing the image pickup tube, so that it is possible to realize a new image pickup
method which could not be realized by the color imaging apparatus using the image
pickup tube.
[0004] When attention is paid to a method of taking out an electrical signal of a one-frame
image as an image pickup method, in the color imaging apparatus using the solid-state
image pickup device, not only the frame pickup method but also the field image pickup
method can be easily realized. In the frame image pickup method, the electrical signal
is produced from the photosensitive section at a frame period. In the field image
pickup method, on the other hand, the electrical signal is produced from the photo
sensitive section at a field period.
[0005] In addition, in the color imaging apparatus using the solid-state image pickup device,
even when the number of pixels is small, an image with high resolution can be obtained
by appropriately setting the kind and arrangement of color filters in the color filter
array.
[0006] A color imaging apparatus according to the first portion of Claim 1 is disclosed
in the document IEEE Transactions on Consumer Electronics, Vol. CE-29, No. 3, August
1983, pages 358-364. In this prior art the two separated chrominance signals are input
into a first and a second processing amplifier, respectively, whose output signals
in addition to the luminance signal are supplied to a color matrix circuit. The processing
amplifiers are controlled by pulses from the synchron generator.
[0007] However, in the solid-state imaging apparatus using the solid-state image pickup
device, the above-mentioned advantage is obtained, butthere is, contrarily, a problem
such that a colorflicker at a frame frequency (30 Hz in an NTSC system) is likely
to occur in the image. As the causes for occurrence of this color flicker, it is possible
to mention, for instance, relative positional deviation between each color filter
and the pixel corresponding thereto, variations in characteristics (trans- mittancy,
spectral factor, etc.) of the color filters, and crosstalk of the stored charges between
the pixels.
[0008] It is an object of the present invention to provide a color imaging apparatus which
can prevent the occurrence of color flicker and the prevention of this colorflicker
can be realized by merely adding a simple circuit to the existing apparatus.
[0009] According to the invention, a novel color imaging apparatus is provided.
[0010] To accomplish the above object, the invention is constituted in a manner such that
a level change for every field is eliminated by controlling the amplitude level of
the separated output of a chrominance signal for every field.
[0011] This invention can be more fully understood from the following detailed description
when taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a circuit diagram showing an arrangement of one embodiment according to
the present invention;
Fig. 2 is a diagram showing one example of a color filter array shown in Fig. 1;
Fig. 3 is a diagram showing the storage state of charges on the n-th horizontal scan
line in the odd fields when the color filter array as shown in Fig. 2 is used;
Figs. 4 to 6 are diagrams showing the storage states of the charges shown in Fig.
3 with regard to the green light, red light and blue light, respectively;
Fig. 7 is a diagram showing the storage state of charges on the (n+1)th horizontal
scan line in the odd fields when the color filter array as shown in Fig. 2 is used;
Figs. 8 to 10 are diagrams showing the storage states of the charges shown in Fig.
7 with regard to the green light, red light and blue light, respectively;
Figs. 11 to 13 are signal waveform diagrams for explaining the operation of a variable
gain control circuit shown in Fig. 1;
Fig. 14 is a circuit diagram showing one example of a practical arrangement of the
variable gain control circuit;
Fig. 15 is a signal waveform diagram showing another example of a gain control signal
of the variable gain control circuit;
Fig. 16 is a diagram showing another example of the color filter array shown in Fig.
1;
Fig. 17 is a diagram showing the storage state of the charges on the n-th horizontal
scan line in the odd field when the color filter array as shown in Fig. 16 is used;
Figs. 18 to 20 are diagrams showing the storage states of the charges shown in Fig.
17 with regard to the green light, red light and blue light, respectively;
Fig. 21 is a diagram showing the storage state of the charges on the (n+1 )th horizontal
scan line in the odd fields when the color filter array as shown in Fig. 16 is used;
Figs. 22 to 24 are diagrams showing the storage states of the charges shown in Fig.
21 with regard to the green light, red light and blue light, respectively; and
Fig. 25 is a circuit diagram showing an arrangement of a color imaging apparatus in
case of using a color filter array shown in Fig. 16.
[0012] An embodiment of the present invention will now be described in detail hereinbelow
with reference to the drawings.
[0013] Fig. 1 is a circuit diagram showing an arrangement of a color imaging apparatus in
one embodiment.
[0014] In the diagram, a reference numeral 10 denotes an image pick-up section. In this
image pick-up section 10, a numeral 11 indicates a color filter array; 12 is a solid-state
image pick-up device; and 13 is a drive circuit of this solid-state image pick-up
device 12.
[0015] The color filter array 11 is an assembled part of color filters which are, for instance,
provided for each pixel (picture element). The solid-state image pick-up device 12
is, for example, the CCD type image pick-up device. This solid-state image pick-up
device 12 consists of a photo sensitive section array and a transfer section. The
photo sensitive section array is an assembled part of photo sensitive sections which
are provided in each pixel. Each photo sensitive section is formed by, e.g., a photo
diode and stores charges responsive to the strength of the incident light. The transfer
section consists of, for example, a transfer section in the horizontal direction and
a transfer section in the vertical direction. Each transfer section is formed by a
CCD.
[0016] The drive circuit 13 allows the charges stored in the photo sensitive section array
to be transferred to the vertical transfer section and drives the horizontal and vertical
transfer sections in accordance with the raster scan, thereby reading out the charges
as the electrical signal from the solid-state image pick-up device 12.
[0017] That is, in the image pick-up section 10, a subject image formed on the photo sensitive
section array as the result of that the light entered through the color filter array
11 is repeatedly horizontally scanned in a manner such that the vertical scan position
is gradually shifted whenever the horizontal scan ends once, thereby fetching the
image as an electrical signal.
[0018] The electrical signal which is obtained in this way is the mixed signal of a luminance
signal SY and a chrominance signal SC. The chrominance signal SC consists of three
signals of a red signal SR, a green signal SG and a blue signal SB. The red signal
SR and blue signal SB among them are the modulated signals in which the signal having
the frequency of one half of the charge transfer frequency in the horizontal transfer
section is used as the carrier wave. In addition, the phase of the red signal SR is
inverted for every horizontal scan line. On the other hand, the green signal SG is
outputted not as the modulated signal but as the original signal (low frequency signal).
The above points will be explained in detail later.
[0019] The above electrical signal is amplified by an amplifier 20 as necessary. An output
signal of the amplifier 20 is divided into two signals and one of these signals is
supplied to a low-pass filter 30 and the other is supplied to a chrominance signal
separating section 40. The low-pass filter 30 extracts the luminance signal SY from
the input signal and supplies it to an output terminal 50.
[0020] The chrominance signal separating section 40 has a band-pass filter 41. The band-pass
filter 41 extracts the red signal SR and blue signal SB from the input signal. The
mixed signal of the red signal SR and blue signal SB which is outputted from the band-pass
filter 41 is supplied to an adder 42, a subtractor 43 and a delay circuit 44. The
delay circuit 44 has a delay amount corresponding to one horizontal scan period (hereinafter,
referred to as 1 H).
[0021] The adder 42, subtractor 43 and delay circuit 44 form a comb filter. The comb filter
receives an output signal from the band-pass filter 41 and delivers the red signal
SR and blue signal SB as output signals. That is, since the phase of the red signal
SR is inverted for every horizontal scan line, by adding, for example, the signal
in the horizontal scan line which is being scanned at present (hereinbelow, referred
to as the current horizontal scan line) which is outputted from the band-pass filter
41 and the signal in the horizontal scan line which was scanned 1H before (hereinafter,
referred to as the previous horizontal scan line) which is outputted from the delay
circuit 44 by the adder 42, the red signal SR is eliminated and the blue signal SB
is obtained. On the contrary, the red signal SR is obtained from the subtractor 43.
[0022] The blue signal SB and red signal SR obtained in this way are respectively detected
by detectors 60 and 70. Thus, the blue signal SB and red signal SR in the low frequency
signal states are derived at terminals 80 and 90, respectively. These low frequency
chrominance signals SB and SR are supplied together with the luminance signal SY which
is obtained at the terminal 50 to a color encoder (not shown) and assembled into a
color television signal.
[0023] The production of the chrominance signal SC in the image pick-up section 10 will
then be explained.
[0024] Fig. 2 is an enlarged diagram showing a part of the color filter array 11. In the
diagram, color filters 111 indicated at W1 to W4 are transparent filters which transmit
the red light R, green light G and blue light B. Color filters 111 indicated at YE1
and YE2 are filters for transmitting yellow and transmit the red light R and blue
light B. Color filters 111 indicated at CY1 and CY2 are filters for transmitting cyan
and transmit the green light G and blue light B.
[0025] Now, when considering the image pick-up in the NTSC system as a typical example,
in Fig. 2, n denotes the n-th horizontal scan line (hereinbelow, referred to as the
n line) in the odd fields OF and n+1 likewise indicates the (n+1)th horizontal scan
line (hereinbelow, referred to as the (n+1) line). Also, n+263 represents the n-th
horizontal scan line (hereinafter, referred to as the (n+263) line) in the even fields
EF and n+264 similarly indicates the (n+1)th horizontal scan line (hereinafter, referred
to as the (n+264) line).
[0026] When the storage states of the charges on the n line are classified on a color filter
unit basis, their charge storage states are as shown in Fig. 3. When the stored charges
are divided with regard to each of the lights G, R and B, they are as shown in Figs.
4 to 6, respectively. Similarly, the storage states of the charges on the (n+1) line
are as shown in Figs. 7 to 10.
[0027] In Figs. 3 to 10, an axis of ordinate indicates a storage level I of charges. Therefore,
now assuming that an axis of abscissa represents a time t, an envelope E of the charge
storage level I will indicate the chrominance signal which is outputted from the image
pick-up section 10.
[0028] In the case where the color filter array 11 and horizontal scan lines are set as
shown in Fig. 2, the red and blue signals SR and SB become the high frequency signals
which were modulated using the carrier wave in which the period that is twice a horizontal
transfer period T is used as one period as shown in Figs. 5, 9, 6, and 10. On the
contrary, the green signal SG is fetched as the low frequency signal as shown in Figs.
4 and 8.
[0029] In addition, in the arrangement as shown in Fig. 2, the phase of the red signal SR
is inverted for every horizontal scan line as will be obvious from the comparison
between Figs. 5 and 9.
[0030] It will be appreciated from the above description that the red signal SR and blue
signal SB are extracted from among the three chrominance signals SR, SG and SB by
the band-pass filter 41 in the chrominance signal separating section 40. These two
chrominance signals SR and SB are outputted in the states whereby they were separated
by the foregoing comb filter. In this case, the respective chrominance signals SR
and SB are obtained due to the addition and subtraction of the signals of two lines
n and n+1, so that the amplitude levels of modulated signals become twice the amplitude
levels IR and lB of modulated signals in one line.
[0031] When the amplitude levels 21R and 21B of the respective chrominance signals SR and
SB which are obtained from the comb line filter are expressed using equations, they
are as follows.
[0033] The first term
in equations (1) and (3) indicates the amplitude level 11 in Fig. 3 and the second
term
denotes the amplitude level 12 in Fig. 7. The second terms
in equations (2) and (4) are the values which will become zero if there are not the
relative positional deviation between each of the above-mentioned color filters 111
and the corresponding pixel, variation in characteristics of each color filter 111,
crosstalk of the stored charges between the respective pixels, etc. Therefore, in
this case, the amplitude levels 21B and 21R become the normal values which are specified
by the first terms
in equations (2) and (4), respectively.
[0034] However, it is fairly difficult to eliminate the above-mentioned positional deviation,
variation in characteristics, crosstalk, etc., so that the second terms in equations
(2) and (4) become the error components of the amplitude levels 21B and 21R, respectively.
[0036] The second terms in equations (6) and (8) are respectively the error components at
the amplitude levels 21R and 21R similarly to the foregoing equations (2) and (4).
[0037] As will be apparent from equations (2) and (6), the signs (+ and -) of the error
components of the amplitude level 21B are the same with regard to both odd fields
OF and even fields EF. Therefore, the amplitude level 21B of the blue signal SB is
the same regarding both odd fields OF and even fields EF. Contrarily, the signs of
the error components at the amplitude level 21R are (+) in the odd fields OF and (-)
in the even fields EF. Consequently, the amplitude level 21R of the red signal SR
varies depending upon the odd fields OF and even fields EF.
[0038] Such a phenomenon occurs even if filters for transmitting green are used in place
of transparent filters W1 to W4.
[0039] The change of the amplitude level 21R of the red signal SR in the odd fields OF and
even fields EF causes the color flicker at 30 Hz in the image.
[0040] Therefore, in this invention, the occurrence of color flicker is prevented by controlling
the amplitude level 21R of the red signal SR which is
[0041] outputted from the subtractor 43 for every field. In the embodiment of Fig. 1, a
variable gain control circuit 100 is inserted between the output terminal of the subtractor
43 and the input terminal of the detector 70. The variation in the amplitude level
21R of the red signal SR which is inputted to the detector 70 is eliminated by changing
the gain of the variable gain control circuit 100 with respect to the odd fields OF
and even fields EF.
[0042] A field index signal FI is used as a signal to control the gain of the variable gain
control circuit 100. This field index signal FI is the square wave at 30 Hz to discriminate
the odd fields OF and even fields EF and is produced by the drive circuit 13. This
drive circuit 13 has a reference signal generator and produces a pulse to drive the
horizontal and vertical transfer sections in the solid-state image pick-up device
on the basis of an output signal of this reference signal generator. At this time,
the field index signal FI is produced for, for example, change-over of the scan of
the odd fields OF and even fields EF.
[0043] The gain control operation of the variable gain control circuit 100 will then be
explained with reference to Figs. 11 and 13. Fig. 11 shows the red signal SR which
is outputted from the subtractor 43. Fig. 12 shows the field index signal Fl. Fig.
13 shows the red signal SR which is outputted from the variable gain control circuit
100. Also, Figs. 11 to 13 show the red signal output in the case where, for example,
a white subject was imaged.
[0044] As shown in Fig. 12, the field index signal FI is the binary level signal of which
the level is inverted for every field. Therefore, if the gain of the variable gain
control circuit 100 is controlled in accordance with the amplitude level of the field
index signal Fl, different gains can be set with regard to the odd fields OF and even
fields EF. Thus, if the set gains for the odd fields OF and even fields EF are preliminarily
properly adjusted, the red signal SR in which the amplitude levels regarding the odd
fields OF and even fields EF are equal can be obtained from the variable gain control
circuit 100 as shown in Fig. 13, thereby enabling the occurrence of color flicker
at 30 Hz to be prevented.
[0045] For instance, a differential amplifier as shown in Fig. 14 may be used as the variable
gain control circuit 100.
[0046] In Fig. 14, numerals 101 and 102 denote transistors forming a differential pair and
103 indicates a transistor forming a constant current source for the transistors 101
and 102.
[0047] The operation of this circuit will now be explained. The red signal SR applied to
an input terminal 104 from the subtractor 43 is supplied through a capacitor 105 to
the base of the transistor 101. After this red signal was differentially amplified
by the transistors 101 and 102, it is supplied to an output terminal 106 from the
collector of the transistor 102 as the signal having the same phase as that of the
red signal SR at the input terminal 104.
[0048] The field index signal FI is supplied to the base of the transistor 103 through an
input terminal 107. Due to this, the collector current of the transistor 103 is controlled
by the amplitude level of the field index signal FI. As will be obvious from the comparison
between equations (4) and (8), the amplitude level of the red signal SR in the even
fields EF is smaller than the amplitude level of the red signal SR in the odd fields
OF. The amplitude level of the field index signal FI is set to a high level in the
even fields EF and to a low level in the odd fields OF as shown in Fig. 12. With the
field index signal at a low level, a gain in the differential amplifier is set to
be equal to a reference gain. In this connection it is to be noted that the gain in
the differential amplifier becomes greater than the reference gain when the field
index signal FI is at a high level. By so doing, the red signal SR on the even field
EF is amplified to a greater extent than the red signal SR on the odd field OF to
permit the amplitude level of the red signal SR on the even field EF to be made equal
to that of the red signal SR on the odd field OF.
[0049] On the other hand, with the field index signal FI at a high level, i.e., with the
even field EF, the gain in the differential amplifier may be set to be equal to the
reference gain. In this connection it is to be noted that the gain in the differential
amplifier becomes lower than the reference gain with the field index signal at the
lower level, i.e., with the odd field OF. In Fig. 14 (+B) denotes a power supply.
[0050] The invention is not limited to the foregoing embodiment.
[0051] For instance, as the gain control signal for the variable gain control circuit 100
which is applied from the drive circuit 13, it is not limited to the field index signal
Fl, but any signal synchronized with the change-over timing of the fields may be used
irrespective of its waveform. For example, as shown in Fig. 15, pulses of a single
polarity (pulses of the positive polarity in the diagram) which are generated synchronously
with the same at which the change-over is made from odd field OF to even field EF
may be used. Even in case of such pulses, for example, if a square wave at 30 Hz is
obtained by providing a monostable multivibrator for the variable gain control circuit
100 and by driving this monostable multivibrator by the above pulses, such gain control
as mentioned above can be performed.
[0052] Also, obviously, the variable gain control circuit 100 is not limited to the current
control type differential amplifier. For instance, it is possible to use a circuit
of an arrangement such that the gain is controlled by switching the load at the cycle
of 60 Hz.
[0053] In addition to the purpose for prevention of occurrence of the color flicker, the
variable gain control circuit 100 may be commonly used in an y correction circuit
which likewise uses a variable gain control circuit.
[0054] Also, the variable gain control circuit 100 may be arranged at the post stage of
the detector 70, thereby controlling the amplitude level of the red signal SR in the
state of the low frequency signal. In this connection, it is to be noted that, when
the differential amplifier as shown in Fig. 14 is used as the variable gain control
circuit 100, it is better to control the amplitude level 21R of the red signal SR
in the state of high frequency signal. Because, it is possible to reduce an interference
of the gain control signal over the red signal due to the frequencies of the two signals
being greatly separated from each other. Even if the amplitude level of the red signal
SR is controlled by the differential amplifier in the state of low frequency signal,
the interference of the gain control signal over the red signal SR can be made as
small as possible through the use of a double balanced differential amplifier as the
differential amplifier.
[0055] The level control means is not limited to the variable gain control circuit. For
example, two circuits with different gains may be used such that they are switched
in dependence upon the odd fields OF and even fields EF.
[0056] The change-over of the above two circuits with different gains and the change-over
of the gains in the variable gain control circuit may be made of effecting an initialization
by external synchronization. In this connection, it is to be noted that the synchronizing
operation so initialized is subsequently carried out by an internal synchronization.
[0057] This invention may be also applied to a color imaging apparatus which permits light
from the subject to be taken therein by means of a color filter array other than that
shown in Fig. 2. In the diagram, color filters indicated at G1 and G2 are filters
for transmitting the green light. The storage state of charges on the n line in this
case is as shown in Fig. 17. On one hand, when the stored charges are classified with
respect to each of the lights G, R and B, the storage states of the charges are as
shown in Figs. 18 to 20, respectively. Likewise, the storage state of charges on the
(n+1) line is as shown in Fig. 21 and the storage states of the charges regarding
each of the lights R, G and B are as shown in Figs. 22 to 24, respectively.
[0058] In a color filter array 11 of the arrangement in Fig. 16, different from the color
filter array 11 in Fig. 2, the chrominance signal whose phase is inverted for every
horizontal scan line becomes the blue signal SB. Therefore, in this case, in Fig.
1, the red signal SR is outputted from the adder 42 and the blue signal SB is outputted
from the subtractor 43.
[0060] As will be obvious from equations (9) to (12), in the color filter array 11 in Fig.
16, the amplitude levels 21R and 21B of any of the red signal SR and blue signal SB
are specified by the color filters 111 which quite differ with regard to the odd fields
OF and even fields EF. That is, in case of any of the red signal SR and blue signal
SB, the amplitude levels 21R and 21B are specified by the color filters 111 indicated
at numerals W1, G1, YE1, and CY1 in the odd fields OF. On the contrary, in the even
fields EF, they are specified by the color filters 111 which are indicated at numerals
W2, G2, YE2, and CY2. This means that the amplitude levels 21R and 21B of both red
signal SR and blue signal SB change in dependence upon the odd fields OF and even
fields EF. Therefore, in this case, it is necessary to control the amplitude levels
21R and 21B of both red signal SR and blue signal SB for every field. This problem
can be solved by providing a variable gain control circuit 110 also on the output
side of the adder 42, for example, as shown in Fig. 25.
[0061] This invention can also be applied to a color imaging apparatus using the image pick-up
tube.
[0062] As described above, according to the invention, since the amplitude level of a separated
replica of the chrominance signal is controlled for every field, the occurrence of
the color flicker at 30 Hz is suppressed to a minimum possible extent. Due to this,
the image pick-up section can be easily manufactured and this can contribute much
to an improvement in yield.